I Expanding universe needs a big bang?

[CHOP]

i have read that it is generally thought to be a consequence of the big bang (so, matter in motion) + something (dark energy) making that matter accelerate. why is the big bang needed in it? can't you just have acceleration?
by the way, i am not sure what is intended by 'prefix' beside the title i used for this post. advanced for advanced responses, i assumed.
thank you

 

[mathman]

What do you start from?

 

[CHOP]

Okay, but then, why don't we include "big bang" when accounting for the Earth circling the sun? The Earth can't circle the sun unless everything 'started' in the first place.
I don't see how the big bang is a contributor to this expansion, because there seems to be nothing to it except that it is accelerating. I mean, the acceleration seems to be not a separate issue to expansion happening (it's one and the same thing). Am I not noticing something?

 

[Grinkle]

"A" means you want answers that are at the graduate or post-graduate level. I am doubtful that is what you want.

Does the discussion in this thread help?

https://www.physicsforums.com/threads/how-can-an-expanding-universe-look-homogeneous.966572/

 

[CHOP]

Hi Grinkle, thanks for the clarification. Well, we'll see how it goes.

 

[CHOP]

Grinkle, oh sorry I didn't see what you were linking to. I'll check it out and thank you

 

[Bandersnatch]

Big bang is the hot and dense early state of the universe that one arrives at when one extrapolates expansion backwards in time. It should not be construed as some ad-hoc creation moment, as it often is depicted in the media, but a straightforward consequence of expansion.

If today the universe is expanding, diluting and cooling, then as you roll back the time you see it contracted, denser and hotter. The somewhat arbitrarily defined earliest (densest and hottest) stages are called Big Bang. Note, that is different than the Big Bang singularity, which is not a physical thing.

 

[CHOP]

Bandersnatch, I can appreciate all that you have said.
But I don't see how the 'bang' is helping explain what we are seeing today. We are seeing separations increasing in an accelerating way.
I can see how a 'bang' would account for matter moving (radially outward), but at constant velocity. Hence the need for 'dark energy' (something to account for the non constant velocity).
I was thinking about this 'acceleration' feature, and it seems that this feature (all by itself) is sufficient to account for what we are observing. That is to say, it seems that this feature may not be a feature of expansion, but be what expansion is.
Hence my post, asking what exactly is the need to account for expansion with dark energy + big bang, as opposed to simply dark energy?
Grinkle, thanks again. I read that other thread and I do not find it resolves my question.

 

[Bandersnatch]

There's a lot that could be said about it, but just consider this for now: for approximately half of its history, the universe was decelerating. This is due to how matter and radiation content acts gravitationally on itself, causing recession to slow down. For objects to move at constant velocity, the universe would have to be empty - and it obviously isn't.

There would never be any universe as we know it if there was no initial impulse to expand, as the retarding effect of high density of matter and radiation in the early universe would always overcome the relatively weak accelerating influence from dark energy.

This is easy to appreciate once one takes into account how various densities scale with changing size (aka scale factor, $a$) of the universe. The matter density changes as $1/a^3$, radiation density changes as $1/a^4$, while dark energy density (assuming one, likely, type at least) doesn't change. The first two cause deceleration, the last one causes acceleration.
Today dark energy is just about a bit stronger than matter (and radiation is negligible). It doesn't take much reduction in scale factor to have matter dominate, and with sufficient reduction radiation becomes dominant, both acting to decelerate expansion. So the early universe had to expand at a really high rate to get to where we are.

I.e. some initial expansion is necessary. Furthermore, it is possible to make a stronger argument, that the initial expansion (at whichever time we define the 'initial' to be) had to be in a narrow range of magnitudes, as too much would shut down nucleosynthesis and formation of structures (galaxies etc.) too soon, and too little would lead to quick recollapse - meaning there would be no CHOP to ask such questions beyond either boundary.

 

[phinds]

I can see how a 'bang' would account for matter moving (radially outward), but at constant velocity.

Well, yeah, if "bang" meant what you seem to think it means, which it does not. The "bang" was not an explosion from a point and in fact the "Big Bang Theory" says nothing about any "bang" but rather just that the universe at the first movement when we can speak meaningfully about it it was a hot dense plasma of possibly infinite extent, with things moving farther apart from each other.

 

[Drakkith]

But I don't see how the 'bang' is helping explain what we are seeing today. We are seeing separations increasing in an accelerating way.
I can see how a 'bang' would account for matter moving (radially outward), but at constant velocity. Hence the need for 'dark energy' (something to account for the non constant velocity).
I was thinking about this 'acceleration' feature, and it seems that this feature (all by itself) is sufficient to account for what we are observing. That is to say, it seems that this feature may not be a feature of expansion, but be what expansion is.

Unfortunately it is not. The way that objects recede from each other in the expansion of the universe is not like the expansion of fragments away from an explosion. The only way for the two to be the same would be for the big bang to have taken place in a single location, and for that location to have been exactly where our galaxy is located at now. That's a very unlikely coincidence.

 

[CHOP]

Bandersnatch, agreed about no constant velocity (as you say, gravity slowed recession. For constant velocity space would need be empty). I mentioned constant velocity because I was showing the only way I could fathom the big bang (BB) as actively contributing to this expansion now. That is, I don't see BB as yielding the expansion now. Especially not when we have a thing called dark energy for it.

The 'initial expanding impulse' you mentioned is (I assume) just what BB is. And since it was 'initial' it is no longer active (correct me if wrong). You mention that the early universe had to expand at a really high rate to get to where we are. I assume you mean the BB, the impulse (it got us 'past' where gravity would dominate the situation). That makes sense to me. But this rate is over now, correct? History. So, here we are, and we have an expanding universe (thanks to BB getting us expanded enough to see what dark energy is up to), whereby the separations increase in an accelerated way.

What does BB have to do with today's phenomenon? BB seems to just have 'prepared' it for dark energy to have go, but is not contributing to it. Or maybe you will say it is contributing to it. Okay, but then I ask: What evidence is there for that? Is there any aspect of the expansion today that can't be accounted for by dark energy, and must be accounted for by BB (a BB that is still active)? Or is the reason we talk about BB only because when we trace back we get to BB, so we just 'kept it alive' (so to speak) tracing forward?

Perhaps I do not have a good understanding of what a BB is, as some have mentioned. Very likely I don't. If that's the problem, then can someone tell me how to think (just generally) of the BB, such that I would obtain a sense of how BB could be one of the two contributors to the expanding activity of today? Put another way, if we remove the dark energy contributor (suddenly like a switch), what exactly would we, on that instant, be seeing, as far as the BB contribution?

Thanks for all your help so far,

 

[Drakkith]

The 'initial expanding impulse' you mentioned is (I assume) just what BB is. And since it was 'initial' it is no longer active (correct me if wrong).

Note that the expansion is due to geometry according to general relativity, not a force. So this 'impulse' should be a setting up of the initial conditions of spacetime that result in expansion, not a 'push' away from everything.

 

[CHOP]

Drakkith,
right, that's how i am currently understanding it. thanks. the reason i made this post was to find out what is motivating physicists (assuming i have not been misinformed of that motivation) to account for today's expansion with: dark energy + bb (as opposed to dark energy only). Why 2 things, not 1? It seemed to me that dark energy (the accelerating 'causer') is all that is needed. If anyone sees that I am missing something (that I ought to have BB included as a secondary contributor), please let me know and thank you again,

 

[phinds]

I recommend the link in my signature

 

[CHOP]

Thanks phinds will check it out now.

 

[Ken G]

It is certainly true that for the last few billion years, dark energy has been the primary driver of expansion. However, as mentioned already, there needs to be a history that sets up for this possibility, and that's what the "Big Bang" describes. So if you are imagining that the Big Bang kind of "hands off" the expansion to dark energy, perhaps a bit like a runner in a relay race who hands the baton to the next runner, then that's probably not a bad way to think about it. But when you watch a relay race, do you not say that all the runners contribute to the victory in the race? So that's what we mean by the Big Bang contributing to the expansion we see today. (It also set the stage for other key aspects, like the ratio of hydrogen to helium that we see, and the presence of the cosmic microwave background, both of which were present prior to the phase where dark energy started to take over, and indeed we needed the Big Bang to explain those observations even before we knew about any need for dark energy).

Perhaps it would help to better understand how the "handing off of the baton" occurs. Dark energy is a property of empty space, but it is fighting the effects of normal gravity. Hence, dark energy can only dominate over normal gravity (which decelerates expansion as mentioned above) once the matter gets spread out enough that there is enough "empty space" to start making dark energy important. Without that early expansion, dark energy would never come into play, just like the "anchor leg" runner in a relay race would never come into play if the first runner did not hear the starting gun and never began the race in the first place.

 

[phinds]

@Ken G I think your explanation is slightly misleading because you leave out the fact that the expansion of the universe has been chugging along since the beginning, regardless of dark energy. Dark energy is not what caused the expansion or the continuing expansion, it's what causes the ACCELERATION of that expansion. The expansion itself is just a continuation of the early expansion

 

[Ken G]

Actually I think that's exactly what my "relay race" analogy makes clear. But I was adding a second paragraph as you posted, it might help. It all depends on what the nature of the issue that CHOP is having, in my view it centers on what he/she interprets "contributing" as meaning.

 

[CHOP]

Ya this is a great, really polished article. Good stuff. The first way expansion was explained to me, was on a beautiful remote canoe trip, sitting on a high rocky island middle of a large pristine lake, silent. I was young. The baking blueberry muffin was used. Where the berries move from center muffin (but of course no center in space) at the relative rates they do. Good memories. By the way I joined this forum today and I am so impressed by it. So simple and intuitive in its display and navigation, and community outstanding. Friendly, so prompt (wasn't expecting that) and solid, careful, info. Great to have joined. I have one problem with only one thing in the article I'll write it up so I can get it cleared. Thanks again,

 

[CHOP]

Ken, on glimpse of you post it seems to be 'communicating' with my thinking. I will respond to it after the section of the article I am not jiving with well:

"After inflation, the universe settled down to a more sedate rate of expansion and THIS is what is normally meant by the EXPANSION of the universe"

This 'settling down' took time right? (to be as settled as it is, from as unsettled as it was). So, the expansion of today is that settled down rate, still setting down or finished settling. Which one is it? What is the rate we are now settled to? How was it determined?

So dark energy (DE) is sort of 'hiding' this expansion, making it not knowable via observation alone. Because we can't tell (via observation) what component of the acceleration is DE, and what component expansion. So there must be some disagreement in numbers which causes the conclusion of the existence of DE, correct?

If that's the case then it's a done deal.
But if nothing forces that conclusion, I am left with a feeling that there is too much going on here. Why can't BB (expansion) be all there is? I assume it's because it is thought to be settling or settled (which is not what we see up there). Okay, but then, how did we determine it was settling? If Gravity is why it was settling, why not it pick up now (gravity is less now)?

Can't whatever explains the 'expansion impulse' (I assume it's an accelerated rate) still be present, just no longer settling due to Gravity's role being negligible? Sort of like a bungee jumper accelerates down and then starts settling, but the line breaks during the settling, so she accelerates again.

The problem I see is: The jumper decelerates after acceleration because elastic tension occurs late; in universe case, tension (gravity) does not occur late, since it's always there, so analogy fails. But what explains why the initial gravity didn't bind against the impulse expansion (immediately)? Wouldn't that explanation 'stand for' her initial free fall?

Thanks

 

[CHOP]

Ken G,
right, so if it's a hand off then that's in line with what i was thinking and posted. i think you may have hit the nail when you said depends on meaning of contribution. it appears that i may have been interpreted as meaning 'has nothing to do with anything at all'. i understand that we need a few separations for anything to get rolling (whether gravity or DE). But in that sense of 'contribution', why don't we say that BB contributes to any other motion in the universe? We don't mention it, hence (since mentioned) I took 'contribution' to mean 'actively facilitating' this expansion, which i found suspicious. So just to clarify then, the expanding universe today (which is an accelerating one) is being 'caused' by DE, and by nothing else. Correct? And since one poster pointed out that DE is a property of relativity (unless I misunderstood), we'll say that the expanding universe happens for no mystery. If false (there is mystery), what is the mystery?
Thank you for your help

 

[weirdoguy]

the expanding universe today (which is an accelerating one) is being 'caused' by DE, and by nothing else. Correct?

Accelerated expansion is caused by dark energy, not the expansion itself. Universe would expand even without it.

 

[CHOP]

weirdoguy, I don't see how what you said up to your comma is different from what I said up to my comma. For the rest, it seems that you are emphasizing: "although there is accelerated expanding going on right now, there is also something else hiding inside of it, and it is a non accelerated expanding"

If that is what you mean, I do not see what motivates thinking it. If you don't mean that, but mean "There was a non accelerated expanding that happened, called the Big Bang", then okay (I know that's the consensus) but shouldn't the Big Bang have been an accelerated expanding?

 

[phinds]

... shouldn't the Big Bang have been an accelerated expanding?

Why would you think that? If you set off a stick of dynamite in outer space would you expect the casing pieces to accelerate as they left the center of the blast? What force is it that you think would create that acceleration?

 

[Drakkith]

weirdoguy, I don't see how what you said up to your comma is different from what I said up to my comma. For the rest, it seems that you are emphasizing: "although there is accelerated expanding going on right now, there is also something else hiding inside of it, and it is a non accelerated expanding"

You're missing the math here. In GR, dark energy is another term that, if adjusted, results in a acceleration of the expansion. Remove that term, and the expansion still occurs. It just doesn't accelerate.

 

[Bandersnatch]

i understand that we need a few separations for anything to get rolling (whether gravity or DE). But in that sense of 'contribution', why don't we say that BB contributes to any other motion in the universe? We don't mention it, hence (since mentioned) I took 'contribution' to mean 'actively facilitating' this expansion, which i found suspicious. So just to clarify then, the expanding universe today (which is an accelerating one) is being 'caused' by DE, and by nothing else. Correct?

I think the main issue here is that it is unclear what, exactly, do you mean by 'being caused'. It is then hard to tell if there is some misconception regarding the expanding universe that needs to be addressed, or if there's a conceptual agreement and we're all just talking past each other.
Let me try and use an analogy to show what I think you're saying and why that would get you the answers you're getting.

In some sense, expansion of the universe is not that different from Newtonian motion of objects in gravitational fields (projectiles, satellites etc.). So let's use one such object - one of the Pioneer probes - as a toy model of the expansion.
The probe represents some generic galaxy we've been observing and which is close enough not to worry about light travel time delays.
The Pioneer moving away from Earth represents the expansion of the universe. Its current velocity represents the recession velocity of the generic galaxy.
As you may remember, there was that issue with 'Pioneer anomaly', which was basically a small, previously unaccounted for acceleration outward due to some asymmetric heat dissipation or other effect (irrelevant here). That small acceleration of the probe - which we'll assume here to be just like a small, magical engine with endless power source, so that we don't worry about it running out of juice - represents dark energy.
The initial velocity imparted to the probe at launch represents the big bang 'impulse'.
Again, for simplicity, we assume there were no fancy manoeuvres, gravity assists or curved paths, just that initial velocity imparted once, and the small 'engine' working constantly as the probe climbs up the gravity well of the solar system.

With that setup in mind, imagine we're taking a look at the probe (=galaxy) today.
It will have some total velocity away from Earth (=recession velocity). This velocity will be a sum of the initial velocity, minus the velocity lost over time due to the gravitational pull of the Sun, plus the velocity gained over the same period thanks to the 'engine' thrusting forward.
Depending on how long after launch (=after big bang) we're looking at the probe, it may still be decelerating, or already accelerating (because the gravity of the Sun fell below the thrust of the engine).
If we look at the probe after a really, really long time, it will turn out that whatever its velocity away will be by then, the majority of its magnitude will have come from the engine. But for a significant amount of time (due to how weak the engine is) the contribution of the initial velocity will be greater. It most certainly is when we look at the actual Pioneers today.

So then, saying that current expansion is being caused by dark energy is like saying the away motion of the Pioneer probes is caused by the anomalous acceleration. After all, that's the only outward acceleration acting.
But, currently, neither the Pioneer anomaly nor dark energy is what contributes the most* to the magnitude of the recession velocity, so saying that it's the cause is not informative.*to illustrate the point, this is what the calculated history (i.e. changes over time) of the recession velocity of a generic galaxy looks like:

(the galaxy is chosen so that its current recession V = c)

Without dark energy it would look something like this:

It's perhaps visible that DE makes a difference, altering the shape of the curve, and the impact is greater the older the universe gets, but the recession velocity would still be close to where it is today without it.
So just as, hopefully, one wouldn't say the expansion depicted on the second graph is caused by DE (because it's not even there), one should similarly not say that DE is the cause for the first graph. What it causes is mostly just that later upswing in velocity.
Just by eyeballing the graphs, and keeping in mind the second one is not 100% kosher, the relative contributions look like 4:1 in favour of the initial impulse (this proportion will change with time in favour of DE).

All those words just to say what I'm guessing you're thinking is.
I.e. that you perhaps assumed the contribution of dark energy to the recession velocities is already greater than the initial impulse, which would, intuitively, justify saying that's the cause, of current state of motion of the galaxies. If not exclusive, then at least the main one.

And if I'm guessing wrong - I'm pretty sure this is the framework for what is or isn't the cause everyone had in mind when they were responding in this thread, so at least look at the responses through these lenses.

 

[Ken G]

Ken G,
right, so if it's a hand off then that's in line with what i was thinking and posted. i think you may have hit the nail when you said depends on meaning of contribution. it appears that i may have been interpreted as meaning 'has nothing to do with anything at all'. i understand that we need a few separations for anything to get rolling (whether gravity or DE). But in that sense of 'contribution', why don't we say that BB contributes to any other motion in the universe? We don't mention it, hence (since mentioned) I took 'contribution' to mean 'actively facilitating' this expansion, which i found suspicious. So just to clarify then, the expanding universe today (which is an accelerating one) is being 'caused' by DE, and by nothing else. Correct? And since one poster pointed out that DE is a property of relativity (unless I misunderstood), we'll say that the expanding universe happens for no mystery. If false (there is mystery), what is the mystery?
Thank you for your help

I think the issue you are having is that the entire concept of "cause" is quite subtle in physics in general, and in dynamics in particular. We don't even need general relativity to discover the issue, it was there in good old Newtonian mechanics. The key law is F=ma, so we say that forces cause acceleration, but we do not say forces cause motion, at least not by themself, because if all you know is the force, it isn't ever enough. You also have to know the initial condition-- how fast was the object moving when you started the calculation? So when we see something moving, which was the cause of the motion-- the forces on the object, or the motion it had initially when those forces began to apply?

The answer is, both. If we cannot assume the object started at rest (and in general we cannot assume that, such as when you see a home run flying off a baseball bat, you should not assume the baseball was at rest when the bat hit it), then neither the forces we are tracking, nor the initial motion, can by themselves tell you the subsequent motion, you need both. So it is for dark energy and the Big Bang-- we think of the Big Bang as being akin to an initial condition (that is not explained by any physics at present), and then the physics kicks in and tells us what happened next. So which is the "cause" of the expansion we now see? Which is the cause of a home run, the batter who hit it or the pitcher who pitched it? Both are, together. That's why the people who make the big bucks in baseball are the pitchers who are not involved in a lot of home runs, and the batters who are.

 

[CHOP]

I will read everyone's posts and thank you all very much for your help and for you patience.

Why would you think that? If you set off a stick of dynamite in outer space would you expect the casing pieces to accelerate as they left the center of the blast? What force is it that you think would create that acceleration?

Hi phinds, Thanks for trying to clear this. I said 'acceleration' because earlier I said I don't see how BB is needed to explain this current expanding phenomenon, since all there is is acceleration, and (as I thought) BB would only yield constant velocity (on the same reasoning you showed with dynamite).

In response, someone said there can't be constant velocity, which I ultimately took to mean that I don't understand BB (it yields non constant velocity (not what I thought)). Further posts (and article) seem to hold: BB is powering today's expanding phenomenon along with DE (strengthening my impression that 'BB = non constant velocity)'.

So let's clarify. Am I fine to think (as I always did) that DE is explicitly the reason for the expanding we see today. And BB is not in any way making that activity happen? Right now, the only thing I can imagine (as far as BB explaining the expanding today), is that BB gives direction to the acceleration. Giving direction is not a form of expanding - not the occasion of expanding. Is that what everyone is meaning to say?

I understand that DE is purely acceleration; that was what my original two comments were based on. I was thinking that this function itself yields expansion (nothing else needed). For example, a line of grains of sand pointed at and close to a large mass. Acceleration (alone) causes those grains to separate, causes their separations to expand. Purely acceleration yields that expanding effect. Hence my suspicion about the notion that BB is (even in part) explaining what we are seeing.

Is there some mistake in how I am thinking? What is the mistake?

 

[PeterDonis]

Moderator's note: Moved post to Cosmology forum.

 

[PeterDonis]

why don't we include "big bang" when accounting for the Earth circling the sun?

Because the solar system is not expanding.

what exactly is the need to account for expansion with dark energy + big bang, as opposed to simply dark energy?

Because dark energy by itself would not have accelerated the expansion to the rate we observe today in the time available. (In fact, as others have commented, dark energy has only been accelerating the expansion for a few billion years; prior to that the expansion was decelerating, because the matter was denser back then and its effect outweighed the effect of dark energy.) There had to be rapid expansion already at the time of the big bang, 13.7 billion years ago, for the expansion today to be what we see.

 

[PeterDonis]

Am I fine to think (as I always did) that DE is explicitly the reason for the expanding we see today. And BB is not in any way making that activity happen?

No. You're leaving out inertia.

13.7 billion years ago, the universe was very hot, very dense, and very rapidly expanding. That very rapid expansion has a huge inertia associated with it--all of that very hot, very dense matter and radiation expanding very rapidly won't just stop, even if no other causal influence is present. It will keep expanding. This effect can be thought of as the big bang still "contributing" to expansion even today.

There are two other causal influences involved, whose effects are combined with the inertia effect I just described. First, the ordinary matter (including dark matter) and radiation (negligible now, but not in the early universe) has attractive gravity, which acts to decelerate the expansion. Second, dark energy (dominant now, but wasn't before a few billion years ago) acts to accelerate the expansion.

The actual expansion profile we observe is the combination of all three of those effects. No one or two of them in isolation will explain it; you need all three.

 

[CHOP]

Because dark energy by itself would not have accelerated the expansion to the rate we observe today in the time available. (In fact, as others have commented, dark energy has only been accelerating the expansion for a few billion years; prior to that the expansion was decelerating, because the matter was denser back then and its effect outweighed the effect of dark energy.) There had to be rapid expansion already at the time of the big bang, 13.7 billion years ago, for the expansion today to be what we see.

Yes, I understand that there has been deceleration due to gravity, and (thus) a rapid expansion was needed to overcome gravity.

I was asking why BB is needed to explain today's activity of expanding, not why a BB was needed to get us to where we are.

Also, as per "inertia" (in your second post), I am perfectly fine with inertia in this. The inclusion of inertia does not prevent what I am asking.

I apologize for the unclear wording of my question. See my previous comment for clarification of where my thinking is, and thank you for your comment.

 

[rootone]

A 'Bing Bang' is not something presumed to be true.
Originally that term was used by Fred Hoyle. as a sort of derogatory remark.
Hoyle was convinced of a steady state Universe.
The fact is though, that generally relativity says a steady state Universe is very improbable.

A 'big bang' means that the math we are sure of ends up with a singularity,
but infinite density does not make sense.
So really, we don't know what was going at all beyond some point back in time.

 

[PeterDonis]

I was asking why BB is needed to explain today's activity of expanding

But since you don't accept the answer I already gave to this question...

The inclusion of inertia does not prevent what I am asking.

...then I'm confused about what you think the problem is. Nobody is claiming that "the big bang" is something that is happening right now. It happened 13.7 billion years ago. So I don't know where your idea that "BB is needed to explain today's activity of expanding" is coming from, if it isn't just the point I've already made about inertia. Where are you getting this from? Can you give a reference?

 

[CHOP]

PeterDonis,

My original question was: Why is the big bang needed to explain the expanding universe?

When I asked that, I was not talking about the expanded universe. I was talking about the expanding universe.

And an expanding universe is one happening now.

Your position seems to be: my question is based on false premise. That is, the big bang is not needed to explain the expanding universe.

That ends the story if that's the consensus. My impression (of what the consensus was) was simply wrong.

I didn't get to know that until you made the post you just made. So thank you very much.

So, its DE. That's how I have always been thinking.
And it was said here that DE is a property of GR, so it is not a mystery at all, correct? If incorrect, what is the mystery?

 

[PeterDonis]

My original question was: Why is the big bang needed to explain the expanding universe?

And the answer is what I've already said: if you don't include the inertia from the big bang, you will make an incorrect prediction for what the expansion rate should be now. So since explaining the expanding universe means explaining why the expansion rate now is what it is, you need to include the big bang in the explanation to get the right answer.

If this answer does not satisfy you, then I think you're confused about what question you're asking, because the answer I've just given is the answer to the question quoted above.

Your position seems to be: my question is based on false premise. That is, the big bang is not needed to explain the expanding universe.

No, that's not what I'm saying. See above.

That ends the story if that's the consensus. My impression (of what the consensus was) was simply wrong.

No, it wasn't. See above.

So, its DE.

Not by itself. See above.

 

[rootone]

Observations are that the Universe is expanding,
It is expanding even quicker than we thought it was 50 years ago.
Nobody can say why it is expanding, but it definitely isn't shrinking.

 

[PeterDonis]

it was said here that DE is a property of GR, so it is not a mystery at all, correct?

I'm not sure it's accurate to say DE is not a mystery. It's easy to model using GR (just include a cosmological constant), but that's not the same as understanding why that model works. I don't think we currently have a good understanding of why including DE in the model works.

 

[Comeback City]

Unfortunately it is not. The way that objects recede from each other in the expansion of the universe is not like the expansion of fragments away from an explosion. The only way for the two to be the same would be for the big bang to have taken place in a single location, and for that location to have been exactly where our galaxy is located at now. That's a very unlikely coincidence.

I'm assuming GR rules this possibility out anyways, yes? (regardless of its likelihood, that is)

 

[phinds]

Observations are that the Universe is expanding,
It is expanding even quicker than we thought it was 50 years ago.
Nobody can say why it is expanding, but it definitely isn't shrinking.

That is not correct. It is expanding due to inertia from the original expansion at the end of inflation (assuming inflation is correct). This is well understood.

What is also understood is that the expansion is accelerating. WHY it is accelerating is attributed to "dark energy" which is a thing that we know the effects of but not the cause.

 

[rootone]

That is not correct.

Only if you assume inflation.
Which is mathematically sound, but is not proof of anything.

 

[phinds]

Only if you assume inflation.
Which is mathematically sound, but is not proof of anything.

No, I don't think inflation is required for the universe to be expanding, although without inflation I assume it would have expanded more slowly originally and perhaps contracted again by now. The Big Bang Theory does not include inflation but assumes an expanding hot plasma as its beginning.

 

[CHOP]

PeterDonis

So you are saying that there is no question "Why is the universe expanding?" unless it were asking "What are the list of things which cause the expansion rate to be precisely what it is?"

That's very interesting. I am going to think about that and I will get back to you.

Thank you.

 

[PeterDonis]

So you are saying that there is no question "Why is the universe expanding?" unless it were asking "What are the list of things which cause the expansion rate to be precisely what it is?"

What other question would you map "Why is the universe expanding?" to in a physics forum? Physics is about making quantitative predictions.

 

[PAllen]

Worth stating explicitly:

Assume homogeneity, isotropy, that the universe is not vacuum, and that the simplest form of the Einstein field equations hold (no cosmological constant), then expansion is mathematically required. Further, it follows that there is no upper bound on energy density observed by a comoving geodesic as it followed backwards in time. It is this last feature that is called a ‘big bang’. Thus, the Big Bang in cosmology based on GR is simply unavoidable.

Inflation and cosmological constant are only needed to explain further details of what we observe, e.g. the degree of uniformity in CMB, and the accelerated rate of expansion.

 

[PeterDonis]

Assume homogeneity, isotropy, that the universe is not vacuum, and that the simplest form of the Einstein field equations hold (no cosmological constant), then expansion is mathematically required

More precisely, expansion or contraction (except at a single instant if the universe is closed). We know from observation that the universe is expanding; math alone can't actually tell us that given the assumptions.

It is this last feature that is called a ‘big bang’.

In an idealized model such as you describe, yes, the term "big bang" can refer to the initial singularity. But I think this usage can be confusing, and I think it's better to try to reserve the term "big bang" for the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence.

 

[PAllen]

In an idealized model such as you describe, yes, the term "big bang" can refer to the initial singularity. But I think this usage can be confusing, and I think it's better to try to reserve the term "big bang" for the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence.

I was not referring to the singularity per se. Simply that energy density grow without bound. The singularity is geodesic incompleteness.

 

[PAllen]

More precisely, expansion or contraction (except at a single instant if the universe is closed). We know from observation that the universe is expanding; math alone can't actually tell us that given the assumptions.

True, but an open manifold with contraction is mathematically indistinguishable from one with expansion. The distinction only comes from coordinates assigned. I think entropic arguments can be used to choose with is the correct physical interpretation of the manifold.

 

[PeterDonis]

an open manifold with contraction is mathematically indistinguishable from one with expansion

Yes, fair point, the choice of which timelike direction is "future" is arbitrary, mathematically speaking.